Olefinic compounds are a class of hydrocarbon compounds which have at least one double bond of four shared electrons between two carbon atoms. In part as a result of their utility as feeds for producing desirable products, olefin demand continues to grow, particularly for light olefin such as ethylene, propylene, and butenes.
Steam cracking is a commercially-available technology for producing light olefin from hydrocarbon-containing feeds. Although ethylene is the primary light olefin product of steam cracking, the process can also produce appreciable yields of propylene and butenes. Since steam cracking process conditions are selected to provide a fixed, predetermined feed conversion, ethylene, propylene and butylene yields are substantially constant.
During steam cracking, the feed is pyrolysed in the presence of added steam, which lessens coke yield, e.g., by decreasing hydrocarbon partial pressure. Even with added steam, however, the pyrolysis produces an appreciable yield of coke and coke precursors, and a portion of the coke accumulates in steam cracker furnace tubes.
Accumulating coke leads to both an undesirable pressure-drop increase across the tubes' internal flow path and a decrease in heat transfer to the feed-steam mixture. To overcome these difficulties, at least a portion of accumulated coke is removed from the interior of a tube by switching the tube from pyrolysis mode to decoking mode. During decoking mode, the flow of feed-steam mixture into the tube is terminated, and a flow of decoking fluid is established instead. The decoking fluid, typically comprising air and/or steam, reacts with and removes the accumulated coke. When sufficient coke has been removed, the tube is switched from decoking mode to pyrolysis mode to resume light olefin production. Although periodic decoking mode operation is effective for lessening the amount of accumulated coke, this benefit is obtained at a substantial energy cost. In part to lessen damage to the furnace tubes. e.g., by repeated thermal expansion/contractions, the fired heaters operate not only during pyrolysis mode, but also during decoking mode, even though an appreciable amount of recoverable light olefin is not produced during decoking mode.
In order to increase energy efficiency and improve the yield of light unsaturated hydrocarbon, processes have been developed which carry out the pyrolysis in a regenerative pyrolysis reactor. Such reactors generally include a regenerative thermal mass having at least one internal channel. The thermal mass is preheated, and then a flow of the hydrocarbon-containing feed is established through the channel. Heat is transferred from the thermal mass to the hydrocarbon feed, which increases the hydrocarbon feed's temperature and results in conversion of at least a portion of the feed by pyrolysis. The pyrolysis produces a pyrolysis product comprising molecular hydrogen, methane, acetylene, ethylene, and C3+ hydrocarbon. The C3+ hydrocarbon includes coke and coke precursors. Some coke remains in the passages of the thermal mass, and the remainder of the pyrolysis product is conducted away from the reactor as a pyrolysis effluent. Since the pyrolysis is endothermic, pyrolysis mode operation will eventually cool the thermal mass. e.g., to a temperature at which the pyrolysis reactions diminish or terminate. Pyrolysis conditions can be restored by regenerating the thermal mass during a heating mode. During heating mode, the flow of hydrocarbon-containing feed to the regenerative pyrolysis reactor is terminated. Flows of oxidant and fuel are established to the reactor, typically in an average flow direction that is substantially the reverse of the feed flow direction. Combustion of the fuel and oxidant reheats the thermal mass to a temperature sufficient for carrying out pyrolysis. The reactor can then be switched from heating mode to pyrolysis mode.
U.S. Patent Application Publication No. 2016-176781 discloses operating the pyrolysis mode in an elongated tubular reactor The reference (e.g., in its FIG. 1A) discloses controlling the pyrolysis mode for increased ethylene selectivity and decreased selectivity for coke and methane by establishing a sharp thermal gradient in the bulk gas temperature profile between a region of substantially constant temperature at which the pyrolysis can occur and a substantially constant lower temperature at which pyrolysis does not occur. During pyrolysis, the position of the gradient within the tubular reactor moves inward as the reactor cools, i.e., toward the midpoint of the reactor's long axis. The cooled reactor is then switched to heating mode, during which the gradient moves outward, i.e., away from the midpoint of the reactor's long axis. Although utilizing such pyrolysis conditions results in a coke yield that is less than that of steam cracking, some coke does accumulate in the channel. Advantageously, the reference reports that accumulated coke can be oxidized to volatile products such as carbon dioxide during heating mode by combustion using a portion of the oxidant in the oxidant flow. Energy efficiency is increased over steam cracking because (i) heating is not needed during pyrolysis mode and (ii) heat released by coke combustion in passages of the thermal mass during heating mode aids thermal mass regeneration. Although the process is more energy efficient than steam cracking, the process exhibits significant variations in coke and acetylene yields during pyrolysis mode, leading to difficulties in product separations downstream of the pyrolysis.
Energy efficient pyrolysis processes are now desired which have flexibility to produce a range of light olefin products, but with less variation in coke and acetylene yields.